To identify key factors that control primary production (P.P.) and trigger cyanobacterial
harmful algal blooms (cHABs), we investigated spatio-temporal variations in P.P. in a continuous
weir system in the Nakdong River once or twice a month from April to October 2018. P.P. was
measured through an in-situ incubation experiment using a 13C tracer. Relative proportion of
pigment-based phytoplankton composition was calculated by the CHEMTAX program based
on pigment analysis using a high-performance liquid chromatography (HPLC). P.P. was higher
in spring (1130 1140 mg C m??2 d??1) and summer (1060 814 mg C m??2 d??1) than autumn
(180 220 mg C m??2 d??1), and tended to increase downstream. P.P. was negatively related to PO4
3??
(r = ??0.41, p < 0.01) due to utilization by phytoplankton during the spring and summer when it was
high. The relative proportion of pigment-based cyanobacteria (mainly Microcystis sp.) was positively
correlated with water temperature (r = 0.79, p < 0.01) and hydraulic retention time (HRT, r = 0.67,
p < 0.01), suggesting that these two factors should a ect cHABs in summer. Therefore, to control HRT
could be one of the solutions for reducing cHABs in a continuous weir system.
